In conventional switching regulators, in the case of a high input voltage, circuits are formed with transistors capable of withstanding voltages higher than the input voltage. However, such high voltage transistors are larger in size, lower in current driving capability, and lower in response speed than low voltage transistors, so that it is difficult to form high performance switching regulators with high voltage transistors.
Further, higher efficiency can be attained by using an n-channel MOS (NMOS) transistor, which has better characteristics than p-channel MOS (PMOS) transistors, for a switching element where the flowing current is largest among the elements of a switching regulator and that is required to operate at high speed.
However, in the case of using an NMOS transistor for the switching element of a step-down switching regulator, the drain of the NMOS transistor is connected directly to input voltage, and the source voltage increases to near the input voltage when the NMOS transistor turns on, so that the NMOS transistor needs a gate voltage higher than the input voltage in order to turn on.
In an attempt to solve this problem, such a method is provided that generates a voltage higher than or equal to an input voltage using a bootstrap capacitor and performs on-off control of an NMOS transistor serving as a switching element using the generated voltage. (See, for example, Japanese Laid-Open Patent Application No. 7-222439 and Japanese Patent No. 3775240.)
However, even if an NMOS transistor is used for the switching element, using high voltage transistors for all the transistors of the switching regulator increases chip area and does not improve low response speed.